Design, synthesis, and photovoltaic characterization of centrally extended donor-acceptor polymer donors incorporating phenazine derivatives

Centrally extended phenazine-based acceptor units were synthesized for use as conjugated donor polymers in polymer solar cells. These phenazine-derived acceptor units were synthesized through two distinct pathways. In the first route, 2,3-dichloro-6,7-difluoroquinoxaline underwent a Stille reaction to produce 6,7-difluoro-2,3-di(thiophen-3-yl)quinoxaline, which was further subjected to FeCl₃-mediated oxidative coupling to yield 9,10-difluorodithieno [3,2-a:2′,3′-c]phenazine. In the second route, 1,2-di(thiophen-3-yl)ethane-1,2-dione was converted into benzo [1,2-b:6,5-b']dithiophene-4,5-dione via FeCl₃-mediated oxidative coupling, followed by condensation with 4,5-difluorobenzene-1,2-diamine to obtain 9,10-difluorodithieno [3,2-a:2′,3′-c]phenazine. Polymerization of these acceptors with BDT-Cl in the presence of a Pd (0) catalyst produced two donor-acceptor polymers, PPB-F and PPB, respectively. PPB-F and PPB exhibited excellent thermal stability with decomposition temperatures of 401.95 °C and 387.37 °C, respectively and have strong absorption in the wavelength range of 430–680 nm. Their optical bandgaps were determined as 1.82 eV and 1.83 eV, respectively. Both polymers displayed deeply lying HOMO levels, calculated at 5.42 eV for PPB and 5.57 eV for PPB-F. Devices fabricated and optimized from PPB-F:Y-7 and PPB-F:PC₆₀BM demonstrated power conversion efficiencies (PCEs) of 4.36 % and 3.56 %, respectively.